1. Field of the Invention
The present invention relates to drilling and completion fluids for use in drilling or completing boreholes penetrating subterranean formations and to methods of drilling or completing boreholes employing such fluids.
2. Description of Relevant Art
Choice of a particular type of drilling, completion or fracturing fluid depends on the subterranean formation characteristics, including geologic formations and mineralogy, borehole stability requirements, presence of any abnormal pressure zones in the formation, and any need to prevent underground water pollution. Whenever one of such existing conditions changes, then the entire fluid system may have to be modified based on the new conditions. Such changes in the fluid system are expensive and time-consuming. Moreover, the fluid system that may properly solve an encountered problem may be less than optimum for the rest of the drilling column.
Mud rheology plays a fundamental role in drilling oil and gas wells. If the rheology of the fluid is not appropriate for the formation and physical conditions of the well, the drilling operations may be spoiled with drilling problems such as lost circulation, poor hole cleaning, fracturing phenomena of the crossed formations, and stuck pipe, for example. Some of the main drilling parameters involved are cutting, lifting and hole cleaning efficiencies (resulting both from variation of the velocity profile of the fluid flow, and from variation of the rheological parameters), and the pressure spatial distribution along the well profile. It is extremely important for the drilling fluid to be able to transport cuttings up to the well surface without any restriction in any of the existing annulus sections. Such unrestricted transport depends on many parameters including the geometry of the annulus section, the rotation velocity of the drill string, the rate of drill bit penetration into the formation, the flow rate of the drilling fluid, the cuttings characteristics, and above all the rheology of the used drilling fluid. It is very important to keep a constant limit on the concentration values of the cuttings during the cuttings transport to avoid solid particle deposition inside the well, risking problems of borehole occlusion, bit balling, and drill string sticking during the drilling process.
Although the efficiency of a number of different drilling fluids in transporting cuttings has been reported at values up to 80%, new technical problems arise when drilling deep water and ultradeep wells. Such problems are compounded when the effects of high pressure and temperature are considered. High temperatures can heavily alter (and reduce) the viscosity of a drilling mud or a completion fluid and can enhance the speed of chemical reactions within such mud or fluid. These consequences can in turn result in other consequences such as for example increased dispersion or flocculation of the mud solids with resultant increase in fluid loss properties and change in the thickness of the mud cake.
In fracturing, highly viscous fracturing fluids transport the proppant, but if such fluids are left intact after fracturing, they can effectively plug the proppant pact leading to highly reduced fracture permeability of the formation. Polymers such as guar, which is a naturally occurring material, or hydroxypropyl guar, have been used in aqueous solutions to provide substantial viscosity to fracturing fluids. However, the viscosity of such polymers degrades with increasing temperature and shear, requiring continuous addition of polymer and on-time mixing to maintain the viscosity of the fracturing fluid.
There continues to be a need for more versatile drilling, completion and fracturing fluids and for more efficient methods of using such fluids.
In the method of the present invention, a xe2x80x9cmulti-viscousxe2x80x9d fluid, or a fluid having xe2x80x9cmultiviscosityxe2x80x9d is used for drilling and completion or for fracturing. By being xe2x80x9cmulti-viscous,xe2x80x9d the fluid has enhanced flexibility for use in drilling or completing a borehole penetrating a subterranean formation, or in fracturing the subterranean formation. As used herein, the terms xe2x80x9cmulti-viscousxe2x80x9d and xe2x80x9chaving multi-viscosityxe2x80x9d mean capable of different and controlled viscosities at different locations in a drilling column.
The particular viscosity of the fluid at a given time is controlled by an electrical potential applied (or not applied) to the fluid. The greater the electrical potential applied, the more viscous the fluid will become. Removal or cessation of the potential field causes the fluid to revert to its original viscosity. Thus, the viscosity of the fluid is controlled by applying and increasing or decreasing or removing an electrical potential on the fluid. Such fluids may also be called xe2x80x9celectro-rheological fluids.xe2x80x9d
According to the method of the invention, an electrical current or potential is applied to such fluid to increase the viscosity of the fluid as the fluid is introduced, or after the fluid is introduced, into a borehole penetrating the subterranean formation. The exact amount of the potential will depend on the desired viscosity of the fluid and the formation characteristics such as in situ stress and temperature. The potential may be adjusted and consequently the viscosity of the fluid may be adjusted to suit the purpose of the fluid in the borehole or the formation. Different potentials or no potentials may be applied at different depths of a borehole so that the same fluid may have different viscosities at such different depths even simultaneously.